Antennas are used in a wide range of devices nowadays, in particular wireless communication devices. Antennas are basically transducers converting electromagnetic fields into electric current and voltages and vice-versa. Among these antennas, some reacts dominantly on the magnetic field. This is the case of loop antennas, made of a coil that is usually put in resonance using capacitors.
The antenna features a certain resonance frequency and bandwidth. These systems are based on resonant RLC circuits: the bandwidth of the antenna can be adjusted using resistive elements (R) while the resonance frequency can be adjusted using capacitors (C).
A particular case of devices using magnetic antennas are so-called smart cards with integrated loops. Another technical field in which this kind of antennas may be used is the field of hearing aids.
The above mentionned systems rely on the usually called “Magnetic Induction in Near Field” technology, which is based on the quasi-static magnetic component of the field generated by a coil through which is flowing a sinusoidal current. This is the well-known transformer principle extensively used for a long time, i.e. when a second coil (at the receiver side) is introduced within that field, the magnetic flux passing through that coil induces a modulated current in the winding.
The field generated by a current loop (magnetic dipole) can actually be divided into three basic components: one inverse distance term proportional to r−1, which is called the radiation term, since this term represents the flow of energy away from coil, one inverse square distance term proportional to r−2, and finally one inverse cube distance term proportional to r−3 which is called the quasi-stationary term.
There are terms in both 1/r and 1/r2 which radiate, that is to say they have matching pairs of E and B vectors orthogonal to each other and to the radial vector. The “far field” is however usually considered to include only the 1/r terms, since they dominate at distances much greater that the wavelength.
At short distance from the current loop, the 1/r3 term (near-filed) dominates and is the major contributor. This 1/r3 term in the B field is independent of frequency, which implies that any frequency can be employed in the near-field domain, for a given coil and current, to generate a specified magnetic field at the receiver. In the near-field region of a current loop, the field properties are primarily determined by the source characteristics, and the electric field is much weaker than the magnetic field.
The total power radiated by the loop antenna is however frequency dependent and proportional to λ2, wherein λ is the respective wavelength, such that at a frequency dependent distance of λ/2π, the three basic terms in 1/r, 1/r2 and 1/r3 equally contribute to the total field. This distance is often referred to as the near field—far field boundary. Other definitions of this boundary exist, depending on the perspective and primarily on the characteristics of the medium through which the field is the criterion used to define it: wave impedance, wave's phase front, etc.
At distances larger than λ/2π, the far-field components dominate, the electric and magnetic fields are directly proportional to one another, and the properties of the field depend primarily on the characteristics of the medium through which the field is propagating.
The frequency response of the antennas may vary significantly due to temperature variations, component spreading, e.g. coil and capacitance value tolerances and nature of material constituing the surrounding environment of the antenna. Also it may be required to align the resonance frequency of the antenna to different carrier frequency to cover different RF frequency bands or to align the antenna resonant frequency on a changing carrier frequency, e.g. due to temperature or aging effects, or due to a bad carrier frequency accuracy.